Smooth Pigweed (Amaranthus hybridus (syn: quitensis)) is a dicot weed in the Amaranthaceae family. In Ontario this weed first evolved resistance to Group C3/6 herbicides in 2004 and infests Corn (maize). Group C3/6 herbicides are known as PSII inhibitors (Nitriles) (Inhibition of photosynthesis at photosystem II). Research has shown that these particular biotypes are resistant to bentazon, and bromoxynil and they may be cross-resistant to other Group C3/6 herbicides.

The 'Group' letters/numbers that you see throughout this web site refer to the classification of herbicides by their site of action. To see a full list of herbicides and HRAC herbicide classifications click here.

Greenhouse trials comparing a known susceptible Smooth Pigweed biotype with this Smooth Pigweed biotype have been used to confirm resistance. For further information on the tests conducted please contact the local weed scientists that provided this information.

Genetics

Genetic studies on Group C3/6 resistant Smooth Pigweed have not been reported to the site. There may be a note below or an article discussing the genetics of this biotype in the Fact Sheets and Other Literature

Mechanism of Resistance

The mechanism of resistance for this biotype is either unknown or has not been entered in the database. If you know anything about the mechanism of resistance for this biotype then please update the database.

Relative Fitness

There is no record of differences in fitness or competitiveness of these resistant biotypes when compared to that of normal susceptible biotypes. If you have any information pertaining to the fitness of Group C3/6 resistant Smooth Pigweed from Ontario please update the database.

The Herbicide Resistance Action Committee, The Weed Science Society of America, and weed scientists in Ontario have been instrumental in providing you this information. Particular thanks is given to Francois Tardif for providing detailed information.

ABSTRACT - The recent introduction of Palmer amaranth (Amaranthus palmeri) in Brazilian
agricultural areas may promote several changes on weed management, especially in no-till
systems and in glyphosate-resistant crops, since glyphosate-resistant biotypes of A. palmeri
have been frequently selected in other countries. Therefore, this research was developed in
order to evaluate the glyphosate susceptibility of a Palmer amaranth biotype recently identified
in the State of Mato Grosso, Brazil. For this purpose, glyphosate susceptibility of three
Amaranthus biotypes was compared: A. hybridus var. patulus, collected in the State of Rio
Grande do Sul - Brazil; A. hybridus var. patulus, collected in the State of São Paulo - Brazil;
and A. palmeri, collected in the State of Mato Grosso - Brazil. Dose-response curves were
generated for all biotypes, considering eight rates of glyphosate and six replicates. All the
experiments were repeated twice. Both A. hybridus biotypes were satisfactorily controlled by
glyphosate, demanding rates equal to or lower than 541.15 g a.e. ha-1 for 80% control (LD80).
The A. palmeri biotype was not controlled by glyphosate in any of the assessments and required
rates greater than 4,500 g a.e. ha-1 to reach LD80, which are economically and environmentally
unacceptable. Comparison of the Brazilian A. palmeri biotype to the A. hybridus biotypes, as
well as, to the results available in scientific international literature, led to the conclusion
that the Brazilian Palmer amaranth biotype is resistant to glyphosate..

A previously unknown glyphosate resistance mechanism, amplification of the 5-enolpyruvyl shikimate-3-phosphate synthase gene, was recently reported in Amaranthus palmeri. This evolved mechanism could introgress to other weedy Amaranthus species through interspecific hybridization, representing an avenue for acquisition of a novel adaptive trait. The objective of this study was to evaluate the potential for this glyphosate resistance trait to transfer via pollen from A. palmeri to five other weedy Amaranthus species (Amaranthus hybridus, Amaranthus powellii, Amaranthus retroflexus, Amaranthus spinosus, and Amaranthus tuberculatus). Field and greenhouse crosses were conducted using glyphosate-resistant male A. palmeri as pollen donors and the other Amaranthus species as pollen recipients. Hybridization between A. palmeri and A. spinosus occurred with frequencies in the field studies ranging from <0.01% to 0.4%, and 1.4% in greenhouse crosses. A majority of the A. spinosus × A. palmeri hybrids grown to flowering were monoecious and produced viable seed. Hybridization occurred in the field study between A. palmeri and A. tuberculatus (<0.2%), and between A. palmeri and A. hybridus (<0.01%). This is the first documentation of hybridization between A. palmeri and both A. spinosus and A. hybridus..

The incidence and wide spread of herbicide resistant weeds is a global problem. Over the past 65 years, repeated use of herbicides has resulted in the evolution of resistant weed species. The first resistant species to triazine was discovered in 1970 in the United States. Since then, a large number of weed species has evolved resistance to several classes of herbicide. Currently, there are 334 resistant biotypes, including 190 weed species (113 dicots and 77 monocots) in over 310, 000 fields around the world Common resistant species are Chenopodium album and Amaranthus retroflexus resistant to triazine, Phalaris minor resistant to isoproturon, P. minor and P. paradoxa resistant to diclofop, Echinochloa colona resistant to propanil, Echinochloa crusgalli resistant to butachlor, Eleusine indica resistant to trifluralin, Lolium rigidum resistant to diclofop, Lactuca serriola resistant to metsuljuron, glyphosate resistance to Eleusine indica, Conyza canadensis, Lolium rigidum, and Lolium multiflorum. Multiple weed resistance to more than one class of herbicides with different modes of action has also been documented with many species. Currently there has been increased herbicide resistance to various weed species around the globe. Most common species are Lolium rigidum, Avena fatua, Amaranthus retroflexus, Chenopodium album, Setaria viridis, Echinochloa crusgalli, Eleusine indica, Kochia scoparia, Conyza canadensis, and Amaranthus hybridus..

Amaranthus tuberculatus represents one of the most dramatic cases of weed invasion documented in the midwestern USA. The species is infamous for evolving resistance to multiple herbicides, and predicting whether these resistances may be transferred to widespread weeds of the Amaranthus hybridus aggregate is a matter of epidemiological concern. Here, we explore the patterns of genetic exchange between Amaranthus tuberculatus and A. hybridus in an effort to understand whether allele introgression occurs throughout the genome and if fecundity penalties are associated with genetic exchange. We evaluated 192 homoploid BC1s at 197 amplified fragment length polymorphism (AFLP) loci, as well as two loci associated with herbicide resistance: ALS and PPO. We also assessed the fecundity of each genotype by evaluation of seed production or pollen development. It was discovered that genetic exchange between the species is unidirectional. Whereas A. hybridus alleles transfer with little or no penalty to A. tuberculatus, the reciprocal exchange is significantly distorted and potentially of limited evolutionary consequence. Our previous hypothesis suggesting unidirectional introgression at ALS owing to circumstantial linkage is now modified to account for the more generalized distortion of genetic exchange observed in this study..

A glyphosate-tolerant population of Canavalia ensiformis was collected in a cover crop in citrus orchards in Veracruz (Mexico), where glyphosate had been used for the first time. A susceptible Amaranthus hybridus L. population was collected from a nearby field that had never been treated with glyphosate. Dose-response experiments indicated a glyphosate tolerance ratio [ED50(C. ensiformis)/ED50 (A. hybridus)] of 7.7. The hypothesis of a high level of glyphosate tolerance was provisionally corroborated on the basis of shikimate accumulation in both species. The susceptible population accumulated 6 times more shikimic acid in leaf tissue 96 h after glyphosate application than the tolerant leguminous crop. Two different physiological factors were involved in the glyphosate tolerance of this C. ensiformis population, which were confirmed by [14C]glyphosate, being a lack of penetration of glyphosate through the cuticle of the leguminous plants and an impaired herbicide translocation to the roots and the rest of shoots. This paper reports that two different nontarget site-based mechanisms, limited absorption and reduced translocation, contribute to the glyphosate tolerance found in C. ensiformis..